The recovery of residual energy from exhaust gases, before being discharged to the atmosphere, has been an objective of engineers since the development of the first steam engines. To this end, it has been the practice to provide steam generating plants with combustion air preheaters and economizers (in which the feed water is preheated) both located in the gas passed to abstract hat from the flue gases flowing therethrough. However, it is generally recognized that the amount of heat that can be abstracted from the flue gases by such conventional air preheaters and economizers is limited by the need for keeping the heat exchange surfaces of these units at a temperature above the acid dew point of the flue gases flowing thereover to avoid the objectionable consequences of having the moisture and acids in the flue gases condense on these heat exchange surfaces, see the Herbst U.S. Pat. No. 4,147,135 of Apr. 3, 1979; the Merritt U.S. Pat. No. 3,910,236 of Oct. 7, 1975; the Marchiex U.S. Pat. No. 3,202,134 of Aug. 24, 1965; and the Switzer, Jr. U.S. Pat. No. 3,148,665 of Sept. 15, 1964. In the case of air preheaters, this has always been an especially troublesome problem as the incoming air is much colder than the flue gases and the conventional solution of this problem is to recirculate part of the already preheated air with the incoming air. Conventional arrangements, therefore, extract little, if any, of the latent heat energy contained in the water vapor present in flue gases and the recovery of sensible heat is likewise reduced.
Many fuels such as wood, fuel oil and natural gas generate water from combustion of the hydrogen content or from the initial moisture content. This water is normally discharged to the atmosphere where it becomes apparent as a cloud of steam.
Currently, there is a growing trend to wash coal before combustion to remove ash and sulfur compounds therefrom. This requires that the coal be ground to a fine size to facilitate the washing process. After this step, the coal is normally conveyed to the boiler as a water slurry and fed directly to the boiler in that condition. This excess water must be evaporated in the fire box causing a loss of heat. However, it becomes water vapor in the flue gases and the heat can be recovered by this invention.